A motor drive device in the related art drives a position sensorless three-phase motor by, for example, converting a DC voltage to an AC voltage by means of PWM (Pulse Width Modulation) due to switching of switching elements of an inverter circuit and outputting the resulting AC voltage to a three-phase motor coil. A type of the motor drive device configured as above is known, which reduces a loss by adopting a modulation method by which ON and OFF states of the switching elements of at least one phase are fixed, for example, a two-phase modulation method, as a modulation method used for the PWM (see, for example, Patent Literature 1).
In the motor drive device in the related art as above, however, an applied voltage from a feed source readily fluctuates in a case where, for example, a DC voltage feed source is a storage battery or the like mounted to a mobile object, such as a vehicle. When the applied voltage becomes relatively large, a surge voltage occurring upon switching of a fixed phase in the inverter circuit becomes large. The related art therefore has a problem that the surge voltage exceeds an allowable upper-limit voltage of circuit forming components.